As is well known in the art, electrical insulators for high voltage lines are usually manufactured from porcelain materials which, although having excellent dielectric properties, show a great number of disadvantages, inasmuch as porcelain materials are highly brittle and are subject to fracture into dangerous sharp flying pieces, are difficult to form in close tolerances, require expensive equipment for the processing thereof, as well as high temperature furnaces for curing the pieces molded therefrom, and are unsuitable for encapsulating or for attaching metal members thereto.
The disadvantages shown by porcelain-type electrical insulators were overcome by the advent of electrical insulators manufactured from heavily filled polymeric materials, particularly polyesters and epoxy resins which contain fillers of hydrated alumina particles for providing carbon tracking resistance due to arcing at high voltage, as well as other types of particulate fillers in amounts up to about 70% by weight, such as described in U.S. Pat. No. 4,001,128, which teaches the addition of fillers formed by hydrated alumina, and suggests that the water of hydration of the alumina evaporates to produce a porosity which is reduced, in accordance with said patent, by means of the addition of a minor proportion of silica sand, preferably treated with an adhesive such as silane-type adhesive. The grains of the alumina and silica in accordance with the above patent, for use in manufacture of heavily filled resin compositions for use in the manufacture of electrical insulators, must be very fine and is has been shown that these finely powdered filler materials do not permit the addition of sufficiently high proportions of the filler into the resin or plastics material, whereby it was believed that the amount of inert filler could not be increased to a significantly higher level than the above mentioned 70%, without at the same time degrading the quality of the product, particularly in connection with its mechanical strength and resistance to tracking and the like. The insulators manufactured from this type of filled plastics materials, therefore, never gained reasonable acceptance in the market.
In order to overcome the above mentioned drawbacks shown by the porcelain insulators and by the filled resin insulators, Perry, Elijah R., in U.S. Pat. No. 4,210,774, patented Jul. 1, 1980, teaches the manner of providing a heavily filled polymeric composition having suitable properties for being used in the manufacture of electrical insulators, and including up to 97% of the non conductive inorganic inert non-porous filler particles, preferably constituted by silica which may contain smaller proportions of alumina and preferably pretreated with a silane adhesive, to thereafter be incorporated into a polyester methacrylate resin in order to form a slurry which may be molded and cured in order to form an electrical insulator.
However, the manner of forming a composition having such high loading, in accordance with Perry, was possible only through the use of appropriate resin precursors which are sufficiently flowable at such high loadings and also more importantly, by grading the filler particles to provide a low void space.
More particularly, Perry describes a composition in which at least four different grades of filler material to maximize loading had to be used, in order to minimize the seize of the voids, and completing the admixture by adding a monomer comprising methyl methacrylate or other polyesters which must have a viscosity less than 150 cps at room temperature, preferably from 1 to 50 cps, in order to obtain an adequate flowability of the resin precursor so that the same may completely fill the voids between the particles of the filler material.
More particularly, Perry describes a composition for the filler material which comprises a mix of relatively coarse silica particles, constituted by a major portion passing a No. 20 but not a No. 40 mesh sieve, admixed with intermediate size particles only a minor portion of which will be retained on a No. 40 mesh sieve screen, and with finer particles 98% of which pass a No. 60 mesh sieve screen but not a 400 mesh sieve screen and still much finer particles of the size of for instance, silica flour, the major portion of which passes a 400 mesh sieve screen.
By the above means, Perry obtained a very low void space in the mixed filler material, but also a plurality of very narrow channels through which the flowable monomer had to pass, hence the necessity of using a low viscosity monomer or polymer precursor, the viscosity of which must be below 150 centipoises and preferably from 1 to 50 cps or less.
The addition of the resin material to this filler, with very low proportions of resin as compared to the proportions of filler, was preferably effected in accordance with Perry, by the treatment of the filler with an adhesive material such as those described and claimed by Gaylor (U.S. Pat. No. 3,956,230 patented May 11, 1976) or by Chi (U.S. Pat. No. 4,311,138 patented Jan. 19, 1982), that are either maleic anhydride-type or silane-type adhesives which provide for the adhesion of the resins to the particles of the filler material in an effective manner such as is clearly shown by either Gaylor or Chi.
Although the heavily loaded polymeric materials of Perry have been considered up to the present date to show excellent properties for being used in the manufacture of electrical insulators, the said material devised by Perry shows the important drawbacks that the procedure for homogeneously admixing the four different grades of silica sand for the filer material is extremely difficult to be effected in order to accomplish a true homogenization of the different particles at the same time preventing stratification thereof, whereby this procedure takes a long time and strict control must be exercised on said procedure in order to obtain a homogeneous mixture of the four different grades of silica sand material for also providing a homogeneous polymeric mixture that may be used for molding and curing in order to produce electrical insulators having uniform properties against tracking by arcing, said against mechanical fractures or high temperature cracks that are common in non-homogeneous materials.
On the other hand, the necessity of providing a very low viscosity polymer precursor in order to secure the filling of the voids between the particles of the filler material, generates the necessity of using unpolymerized or only partially polymerized monomers which thereafter take an extremely long time for being cured, whereby the process of molding and curing the composition for the manufacture of electrical insulators is extremely slow, with the consequent increases in the costs of production of said electrical insulators.
Therefore, all the ceramic or porcelain compositions, as well as all the filled polymeric compositions extant in the prior art for the manufacture of electrical insulators left much to desire, whereby for long time it has been sought to produce a composition which is not so difficult to be compounded and the quality of which is not so difficult to preserve, and that may still contain an extremely high proportion of inert organic filler and a low proportion of the polymeric materials, in order to have excellent electrical insulation properties not lower than those shown by the composition of Perry, without any satisfactory results up to the present time.